Automatic gain correction (AGC) is used to compensate for the signal attenuation in a signal transmission path between a transmitter and a receiver, so that the receiver can provide a constant output level in the presence of this varying input signal level.
Conventional AGC circuits typically employ feedback techniques.
FIG. 1 of the accompanying drawings is a block schematic diagram of a simplified radio receiver and illustrates a classical radio frequency (RF) AGC system.
An antenna 10 is coupled to a gain controlled RF amplifier 12. An output of the RF amplifier 12 is coupled to a simplified frequency down-conversion stage 14 in which a received RF signal is frequency down-converted to base band and applied to a signal processor 16 which generates the audio output. The architecture of the frequency down-conversion stage 14 can be of any suitable design known in the art and for the purpose of illustration comprises a superheterodyne stage comprising a mixer 18 having inputs for the output of the RF amplifier 12 and a local oscillator 20, respectively.
A bandpass filter 22 is coupled to an output of the mixer 18 to select the desired signal from the products of mixing. A baseband or audio frequency gain controlled amplifier 24 has an input coupled to an output of the bandpass filter 22 and an output coupled to an input of the signal processor 16.
The purpose of AGC is to adjust automatically the gain of the receiver such as to enable the receiver to deliver an adequate level of signal to the input of the signal processor 16.
In the circuit illustrated in FIG. 1, automatic gain control is applied to the RF amplifier 12 and to the baseband amplifier 24. The signal received at the antenna 10 is a broadband signal shown by the inset drawing (of power versus frequency), the broadband signal includes the desired signal FW and unwanted signals in adjacent bands. The total power received from the antenna at an input of the RF amplifier 12 is Ptot. The power of the amplified broadband signal at the output of the RF amplifier 12 is Pout and this power is detected at the output of the amplifier 12 using a power detector 26. The power detector 26 produces an output Pdet which is applied to one input of a comparator 28. A threshold stage 30 is connected to a second input of the comparator 28 and provides a threshold value against which Pdet is compared. The threshold value is selected to maximise the gain of the RF amplifier 12.
An output of the comparator 28 is coupled to an integrator 32 which has an output coupled to a control input 13 of the RF amplifier 12. In operation, if Pdet exceeds the threshold value then the AGC circuit reduces the gain of the RF amplifier 12 and conversely if Pdet is less than the threshold value the AGC circuit increases the gain of the RF amplifier 12. This process continues with the objective of adjusting the gain until Pdet equals the threshold. In practice, Pout is regulated in order to avoid overloading the stages following the RF amplifier 12, in this illustrated case, the frequency down-conversion stages.
The base band amplifier 24 receives a narrowband signal, shown inset, comprising the desired signal FW and, possibly, residues FR from the adjacent channels depending on the quality of filtering by the bandpass filter 22 and provides an amplified constant level output signal to the signal processor 16.
In order to control the gain of the baseband amplifier, an output derived from the signal processor 16 is applied to a control input 25 of the base band amplifier 24 in order to keep its output constant.
In the case of frequency modulation, the signal processor performs frequency demodulation. In the case of amplitude modulation, the signal processor processes the baseband signal which itself comprises the original signal which was modulated onto the carrier.
The circuit of FIG. 1 is used to illustrate the feedback nature of AGC control loops. This invention relates in particular to amplitude modulated signals, and in particular after the frequency down-conversion of the signal has been carried out. Thus, the invention concerns the baseband gain control.
An issue with feedback circuits of this type is that the settling speed may be compromised by ensuring loop stability. In particular, it is difficult to ensure fast response without oscillation or a large distortion figure. In general, this means that the gain control loop filter can only have a low order.
The filtering that can be implemented in the gain control loop is limited by the desired response time. As the strength of inter-modulation is directly related to the amount of audio content in the case of AM radio, strong filtering is also required in determining the gain.
There is therefore a need for a system in which the filtering required to determine the desired gain can be freely selected, thereby resulting in a system where a low distortion can be ensured. By enabling a freely selectable filter, the bandwidth/speed of the filter can be changed without concern for the loop gain.
This invention relates particularly to amplitude modulation signal broadcast. In this type of system, a dc component is often transmitted with a certain modulation index. This dc component is used to ensure low noise at low modulation.